Generally, a rear derailleur for a bicycle, comprises a main body provided with a base member supported to a hub shaft at a rear wheel; a movable member movable axially of multistage sprockets with respect to the base member; and a changeover frame provided with a pulley plate swingably supported to the movable member. A guide pulley and a tension pulley are supported rotatably to the pulley plate. The changeover frame together with the movable member move axially of the sprockets, so that a driving chain in mesh with each pulley can be switched to a desired sprocket.
The changeover frame has a guard plate provided axially outwardly of and opposite to each pulley to thereby prevent the chain from escaping therefrom.
The guard plate at the rear derailleur which is mounted on a hub shaft of a rear wheel corresponding to the sprockets, is opposite at the outer surface to spokes of the rear wheel. In a case where the front edge of the guard plate, in the forward rotating direction of the rear wheel is open with respect to the tension pulley and a gap larger than the diameter of a spoke is formed between the guard plate and the tension pulley, the following problem is created.
Since the low speed sprocket is positioned in close proximity to the spokes of the rear wheel, when the chain is over-shifted more than required, or switched by the derailleur supported to a fork end of the bicycle frame deformed by an accident where the bicycle is dropped, the spokes of the rear wheel when rotating enter the gap between the guard plate and the tension pulley. As a result, the spokes break or an emergency braking on the rear wheel may cause the bicycle to fall.